Musculoskeletal Imaging: Extremity Imaging Equipment

Bodies are made to move. Unfortunately, not all imaging modalities can keep up. Aches and pains in muscles and joints present a specific imaging challenge because in many cases, issues only arise when a patient moves a certain way or is placing weight on the problem area. Knowing this, researchers are working on imaging techniques that can assess musculoskeletal conditions during the everyday situations in which they painfully make themselves known.

Activating Active-MRI

Active-MRI, a technique developed at the University of California Davis (UC Davis) School of Medicine, may soon enable physicians to investigate dynamic wrist instability in vivo

“While conventional MRI is able to assess static joint stability, it does not assess dynamic instability,” explains Abhijit J. Chaudhari, PhD, assistant professor of radiology at UC Davis.

Conventional MRI isn’t fast enough to make a video of a moving wrist, but CT and fluoroscopy provide their own challenges in that they involve radiation and don’t show soft tissue ligaments as well as MRI.

Unlike a standard MRI exam, which can take more than 30 minutes total and requires at least three minutes per image set, the active-MRI protocol takes one image every half second and can deliver a series of images in 30 seconds, giving it the ability to image wrists in motion with a live-action movie. With such a system, patients can reproduce painful motions while they are inside the scanner.

The outcome of initial tests of the protocol conducted by Chaudhari and colleagues was published last December in PLoS ONE. The initial study involved images of 15 wrists from 10 asymptomatic volunteers. To help eliminate banding artifacts caused by bone movement, the researchers created a harness with dielectric pads to stabilize the magnetic field and shift artifacts away from the area of interest.

Optimizing parameters for fast scanning, the researchers focused on obtaining images with high temporal sampling, with the idea that the high temporal resolution would complement the high spatial resolution typical of MRI, says Chaudhari.

During the 10-minute exam, study volunteers were asked to perform motions such as rotating their wrist or clenching their fist. Chaudhari and colleagues were successfully able to evaluate distal radioulnar joint subluxation ratios, scapholunate intervals and abnormalities in extensor carpi ulnaris tendon translation.

While the paper focused on scapholunate dissociation and distal radioulnar joint instability, Chaudhari says Active-MRI can be used to assess any type of dynamic instability.

“One objective of our work was to show that Active-MRI is feasible with commonly available technology—we used equipment that was released in 2006,” he adds. “The scan does not require advanced scanning capability.”

The next step for the authors will be to validate the technology in a group of patients with signs of wrist instability, as well as use the technique to study sex distinctions in conditions such as osteoarthritis and carpal tunnel syndrome.

Inside dedicated cone-beam CT

While Active-MRI uses existing equipment with a modified technique, sometimes advances in care require new tools built from the ground up. A prototype of a cone-beam CT scanner dedicated to extremity imaging now in clinical studies at Johns Hopkins University in Baltimore has shown promise in performing musculoskeletal imaging, including weight-bearing scans.

“The system allows us to visualize the extremities in volumetric images with sub-millimeter spatial resolution and soft tissue visibility comparable to conventional CT,” says Jeffrey H. Siewerdsen, PhD, who led the initial development in academic-industry partnership between Johns Hopkins University and Carestream Health. “In particular, the scanner allows imaging of the weight-bearing lower extremities, including the foot, ankle and knee.”

The system is based on “cone-beam” CT, which obtains fully volumetric data from multiple projections acquired in a single rotation without moving the patient through the scanner.

“Our studies demonstrate the ability to image both the upper and lower extremities, including weight-bearing configurations that could allow new capability in diagnosis of impingement, dynamic deformities, or malalignment, instability, and joint narrowing associated with osteoarthritis and other musculoskeletal conditions,” says John A. Carrino, MD, MPH, who led the first clinical studies.

Results of an initial assessment, published in the March issue of Radiology, showed the dose for a nominal scan protocol to be 9 mGy, less than standard multidetector CT, with isotropic spatial resolution exceeding 15 line pairs per centimeter. Contrast resolution was sufficient to visualize muscle, fat, ligaments, tendons, cartilage, joint space and bone.

There’s also an economic argument for this type of scanner. Musculoskeletal medicine specialty practices may find it makes more financial sense to utilize a simplified, single-organ device rather than a conventional scanner.

The current prototype provides streamlined workflow and a large field of view to accommodate larger patients. As the project continues to evolve, the researchers say the scope of applications will expand, including the integration of quantitative imaging. The ultimate role of such a system also could be defined by engaging a multi-disciplinary team of radiologists, surgeons, rheumatologists and medical physicists.

“In clinical practice, the device could enable load-bearing volumetric imaging with greater ability to obtain physiologic information to depict malalignment and other conditions,” said Shadpour Demehri, MD, PhD, who heads the clinical studies now underway at Johns Hopkins Hospital. “The system also provides a level of image quality suitable to visualization of soft tissues, including tendons, ligaments, and cartilage  for which the ability to image in a natural weight-bearing state is fairly new and could yield important insights and diagnostic capability of soft tissue abnormalities that can occur or are exacerbated in weight bearing exams.”

Much musculoskeletal pain is the result of being on the move, but thanks to projects like Active-MRI and dedicated extremity CT scanners, imaging may soon have a way to catch up.